He has a cameo at the start of the film. It does take place a few months after the events of "The Chronicles of Riddick".
This is going to be awesome. Twohy really wanted to go back and recapture "Pitch Black" but without losing what was established and expanded in "The Chronicles of Riddick". I think those that did have a problem with the latter film will be very pleased with "Riddick". It really goes back to the basics of the concept. I also get the feeling that Diesel really enjoys playing the character.